design of compact c band high power transmitter · 2014-10-02 · design of compact c band high...

Design of Compact C Band High Power

Transmitter

Venkatesh Prabhu1, NC Saha

1, Raghu Ramudu Chinnem

2 and M Madhava

2

1 LRDE, C.V. Raman Nagar, Bangalore – 560 093.

2 BEL, Bangalore – 560 093.

[email protected]

Abstract

This paper presents the design issues and challenges in the

realization of compact high performance Travelling wave tube

(TWT) based C Band Transmitter. The design goal was to

achieve stable operation of the transmitter having spectrum

purity of near carrier noise of -75dBc/Hz at 100 Hz away from

the carrier while delivering 70 kW (minimum) of peak power

and 3 kW of average power across the bandwidth of 400MHz at

C-Band. The transmitter is realised in a volume of 1.2m3 and

weight of 575kgs. The TWT requirements translates into High

Voltage Power Supply (HVPS) design, i.e., Cathode voltage of -

39 kVDC and Collector voltage of 27kVDC with stringent

cathode pulse to pulse regulation of the order of 0.0004% under

adverse environmental conditions which includes operation at

altitudes of 16000feet. Transmitter system design and approach

followed for the realization of major subsystems like High

voltage power supply, floating deck modulator and control and

protection circuits in a compact volume and weight is discussed

in detail. A FPGA card is used for status transfer and remote

operation.

Keywords: Tapped transformer, Interleaved Boost converter,

High voltage engineering, series resonant converter

I. INTRODUCTION

High Power Radar Transmitters using microwave tubes

like TWT require high quality power at HVDC to obtain

satisfactory performance in terms of RF output spectrum.

Further Radar power supplies are subjected to pulsed load,

which calls for a suitable energy storage capacitor at the

output. Operating HV power supplies under environmental

conditions such as humidity and high altitudes calls for

specific high voltage engineering to prevent corona and

partial discharge effects.

HV transformer is a crucial element in HV power

converters due to large number of secondary turns and

insulation requirements resulting in non-idealities like

leakage inductance and winding capacitances. Series

Resonant converter (SRC) absorbs some non-idealities

with its own advantages and disadvantages. Liquid

dielectric has been in use for HV transformer for a long

time considering its ability to remove heat by convection,

good dielectric strengths and its insulation restoration

properties. A tapped High voltage High frequency

transformer is used in this transmitter to generate both the

cathode and collector supplies for the TWT. Cathode

voltage is regulated by phase modulation of the SRC and

collector voltage is maintained by proper cross regulation

of transformer.

The paper also presents the approach used to reduce the

size of the floating deck modulator (FDM) and control and

protection strategy employed for the transmitter. A FPGA

board is used for status transfer and remote operation.

Schemes employed to ensure reliable operation of the

FPGA board in the transmitter has been described

considering the high EMI/EMC environment and

possibility of high current discharges in the HV section.

II. SYSTEM DESCRIPTION

Some of the important specifications of the transmitter are

given in Table 1.

Parameter Specs

RF Peak Power output 70kW

Duty cycle 5%

RF Frequency C Band

Phase noise -75dBc/Hz @ 100Hz offset

RF input 0dBm

Input Power 3 Phase 415V, 50Hz

Size 600mm(D)X950mm(H)X2100mm(L)

Weight 600kg max

Table 1

The voltage variations from pulse to pulse of the TWT

electrodes and the phase sensitivities of the TWT electrode

voltages contribute to the phase changes from pulse to

pulse and the phase noise in the RF output. Phase noise

performance to a large extent is decided by the regulation

of the cathode supply of TWT. Typical cathode phase

sensitivities for a High power TWT is in the range of 35 to

45 degree/ 1% change in the voltage. To achieve the

required phase noise performance a cathode voltage

regulation of the order of 0.0004% is required in this

transmitter. Since TWT’s can tolerate much higher

variation in collector voltage without degrading the phase

noise performance the High voltage power supply topology

selected is a single phase modulated SRC powering a

tapped high voltage high frequency transformer with

cathode voltage regulated and the collector voltage

maintained by the cross regulation of the transformer.

9th International Radar Symposium India - 2013 (IRSI - 13)

NIMHANS Convention Centre, Bangalore INDIA 1 10-14 December 2013

High voltage power supply, modulator and control circuits

form the major subunits of any transmitter. The approach

for realization of these units in a compact volume is

discussed below.

1. High voltage power supply

A four stage interleaved boost converter with power factor

control (PFC) is used as the preregulator for the inverter.

The boost converter is operated at 20 kHz. The high

frequency pre regulator enables to reduce the size of the

DC bus filter components which occupies the bulk of the

volume in any AC/DC converter. Also PFC control enables

the use of a smaller EMI filter at the input. A 50 kHz phase

modulated SRC and Tapped high voltage high frequency

transformer is used to generate the required cathode and

collector voltages for the TWT. Zero voltage switching

(ZVS) is used to reduce switching losses in the switching

devices (IGBT’s). The transformer has been designed with

Low leakage inductance and good cross regulation to

maintain collector voltage of TWT within acceptable limits

from no load to full load. Sandwich bas bars are used for

providing DC input to the full bridge IGBT inverter.

2. Modulator

Beam switching of pulsed linear beam tubes can be

performed using low power or high power modulator to

switch the beam on and off [4]. Low power modulators

exploit a control electrode such as grid, a focus electrode

or an anode. In cathode modulation, high instantaneous

powers are involved since both the full beam voltage and

current have to be switched simultaneously. TWT’s

generally require a grid voltage swing of ±800V approx for

switching ON and OFF the beam.

Fig 1. FDM unit

Floating deck modulator (FDM) is so called because the

control electrode drive and bias supplies, as well as the

switching devices are floated on the cathode voltage,

which could typically be several kilovolts with respect to

ground. The transmitter system being described here uses a

Floating deck modulator for pulsing the TWT. FDM is

constructed on the principle of Faraday cage.

In order to realize a compact transmitter, realization of

compact FDM is essential as FDM has to be mounted in

the transmitter providing required clearance from the

ground surfaces to ensure that there is no corona initiation

under worst operating conditions. FDM generates the

filament and grid supplies and uses a solid state High

voltage push pull switch (Mosfet based) for pulsing the

TWT Grid as per pulse width and PRF requirements. A

half bridge inverter operating at 30 kHz is used to provide

AC input to the FDM through a high frequency isolation

transformer to reduce the size.

3. Control and Protection

The control and protection unit (CPC) performs three

major functions:

• Control functions like sequencing the switching

on of the transmitter, generation of timing signals

for subsystems and checking the status of

subunits.

• Protection functions include detection of faults,

classifying them according the level of

seriousness and auto switching off the faulty

subunit to prevent destructive damage. It also

displays the nature of fault.

• Monitoring and display of essential parameters.

Quick acting latching comparators are used for

interlocking high voltage and pulse parameters. Fault

clearing time is less than 5us. FDM parameters are

obtained using Voltage to frequency (V to F) and F to V

conversions with data transmitted as light pulses on

optical links. All PCB’s are realized as multilayer boards

with power and ground planes.

A Vacuum florescent display (VFD) is used to display

the transmitter status and parameters. A rugged metallic

keypad is used as user interface for operating the

transmitter. Representation of critical transmitter

parameters, monitoring the transmitter status and health

of the various subunits on the local VFD display and

communication with the radar controller through

Ethernet is achieved using a FPGA board.

Transmitters being an EMI environment following

precautions have been taken to prevent nuisance

operation of protection circuits and FPGA board.

• All samples are taken through RC filters and

terminated with transient absorbs at PCB input.

• All PCB’s are mounted inside grounded

metallic enclosures.

• Single point grounding scheme has been

adopted for the transmitter with ground

impedance of less than few milliohms.

• Small loop area for high current paths in the

transmitter to reduce the inductance effects.

• Isolated power supplies for the FPGA board

• All interface between the transmitter control

circuits and FPGA board through optical

isolation

• High voltage discharge currents are limited to

<500A using suitable current limiting resistors

during arcing and crowbar operation.

• Dedicated ground paths are provided for

carrying crowbar currents



4. High Voltage Engineering

Solid encapsulation techniques have been used in the

transmitter for HV insulation requirements of different HV

components. The breakdown strengths of the dielectric

have been sufficiently derated to ensure reliable operation.

Compact molded High voltage probes are used to obtain

samples for protection and cathode voltage regulation.

Compact HT connectors have been developed to make

EHT interconnections thereby avoiding exposed EHT

terminals.

Fig 2. Solid encapsulated HV components

Solid encapsulated HV components used in the transmitter

are shown in Fig.2. Polyolefin cross linked polymer is used

as the dielectric material with a dielectric strength of

30kV/mm.

III. RESULTS

Compact transmitter has been realized in the targeted

volume of 1.2m3 and a weight of 570kgs, shown in Fig.3.

Currently the transmitter has been tested up to 2% duty

cycle. Agilent E8257D RF source was used to drive the

transmitter with 0dBm input with a near carrier noise of -

62dBc/Hz @ 100Hz offset. The output spectrum was

measured as -60dBc/Hz @ 100Hz offset (Fig 4). Final RF

spectrum measurements at 5% duty cycle will be carried

out with RF input from exciter with near carrier noise of -

82dBc/Hz @ 100Hz offset.

Fig 3. Realized compact C-Band transmitter

Fig 4. RF output spectrum

IV. CONCLUSION

A high performance Compact TWT based

Transmitter using state of art technology for

Weapon locating Radar has been realized. The transmitter

has been tested up to 2% duty cycle and testing is under

progress to complete the 5% duty cycle operation. The

transmitter will be qualified for EMI/EMC requirements as

per MIL-STD 461E and environmental requirements as per

JSS 55555.

REFERENCES

[1] Biju S Nathan, V Ramnarayan , “ Designing for zero-voltage

switching in phase-modulated series resonant converters”, J. Indian Institute of Science, July-Aug 2000, 80, pp 347-361.

[2] Chuanyun Wang, “Investigation on Interleaved Boost Converters and

Applications”, PhD Dissertation submitted to Virginia Polytechnic Institute, July 2009.

[3] Philip C Todd, “UC3854 Controlled Power Factor Correction Circuit

Design”, Unitrode Application note-slua144. [4] L Sivan, [4]“Microwave Tube Transmitters”, Microwave Technology series, Chapman & Hall, 2-6

Boundary Row, London, U K,1994.

[5] Patro,etal, “Low noise High power TWT based Transmitter”, Conference record 20th Power modulator symposium, IEEE, 1992

BIODATA OF AUTHORS

Venkatesh Prabhu received BE degree in Electrical

& Electronics from Regional Engineering College,

Suratkal, in 1999 and ME degree from Indian Institute of Science, Bangalore in 2001. Since 2001

he has been with Electronics & Radar Development

Establishment [LRDE], Bangalore involved in design and development of high power Radar

transmitters. He contributed towards indigenous development and realization of high power radar transmitters using TWT

for 3D Surveillance Radars and Weapon locating Radar. He is recipient of

many republic day awards of LRDE and DRDO Technology Group Award in the Year 2004. He is a recipient of “DRDO Young scientist”

award for the year 2010 and IETE-IRSI Young scientist award for the

year 2011.



N C Saha received MSc degree in Electronics and radio Physics from Burdwan University in 1977

and M Tech degree from Pune University in 1995.

Since 1981 he has been with Electronics & Radar Development Establishment [LRDE], Bangalore

involved in design and development of high power

Radar transmitters. He contributed towards indigenous development and realization of high

power coherent radar transmitters using Magnetron’s, CFA’s, TWT’s for

program like INDRA II PC, Airborne surveillance Platform (ASP) and 3D Surveillance radars. He is the project director for 3 D Tactical control

Radar for Indian Army. He is recipient of many republic day awards of

LRDE and DRDO Technology Group Award in the Year 2004 and 2012. He is recipient of “DRDO Performance Excellence award” for his

contributions to 3 D Surveillance Radar, Rohini.

Raghu Ramudu Chinnem received BE degree in

Electronics & Communication Engineering from Osmania University, Hyderabad, in 1996. Since

1997 he has been with Bharat Electronics Limited

[BEL], Bangalore involved in design and development of high power Radar transmitters. He

contributed towards indigenous development and

realization of high power radar transmitters using TWT for Battery Level Radar (BLR III), Flight Level Radar (FLR), Weapon Locating Radar

(WLR) and Troop Level Radar (TLR). He has made significant contribution towards indigenous development of WLR (Swathi), which

bagged the prestigious Raksha Mantri Award for Excellence 2010-11 for

import substitution. He is recipient of BEL R&D Excellence Award for year 2011-12 under category “Key Contributor” for developing

transmitters for the Radars.

Madhava M received B.E. degree in Electronics

and Communication Engineering from Bangalore Institute of Technology Bangalore, Visvesvaraya

Technological University in 2004. Since 2005 he

has been with Bharat Electronics Limited, Bangalore involved in design & development of

Optronics for Thermal Image (TI) Camera and high

power Radar transmitters for Weapon Locating Radar (WLR). He has made significant contribution towards indigenous development of TI

camera integration with Upgraded Flycatcher Radar, which bagged

prestigious Raksha Mantri Award for Excellence 2008-09 for design effort and made significant contribution in WLR (Swathi) which bagged

the Raksha Mantri Award for Excellence 2010-11 for import substitution.



design of compact c band high power transmitter · 2014-10-02 · design of compact c band high...

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